One typical system of multi-carrier transfer systems that use a plurality of sub carriers to provide multiplex transfer is conventionally known as a multi-carrier-code-division multiplexing (MC-CDM) system. The MC-CDM is supposed to have features that frequency diversity effects can be obtained by means of using orthogonal codes for providing frequency multiplication of the signals spread in frequency direction through a plurality of sub-carriers, resulting in better receiving characteristics of the modulated symbols provided. A problem arises, however, that the receiving characteristics becomes deteriorated because of inter-codes interference that will occur if the orthogonal nature between the codes is damaged by frequency selectivity of a radio channel.
FIG. 13 is a block diagram that illustrates a configuration of a conventional multi-carrier-code-divided multiplexing system. FIG. 13 shows a transmitting device 100 including a modulator (MOD) 101 that modulates transmitting data and outputs modulated symbols. Description is given based on the symbols as b [n] as follows: However, n is a mark showing time, giving a modulated symbol number. As an example of a modulating system, a quadrature phase shift keying quadri-phase shift keying (QPSK) system is used herein. A modulating symbol b [n] can obtain a signal point alignment on an IQ surface (that is a flat surface structuring with values that channel I (real numbers) and channel Q (imaginary numbers) can obtain), as illustrated in FIG. 14. FIG. 14 indicates four standard signal points that the modulating symbol b [n] can obtain.
A spreading and multiplexing unit 102 spreads modulating symbols in code, and multiplexes these modulated symbols. The simplest combination, duple spread (spread ratio is 2, and one modulating symbol is spread into two sub-carriers) and duple multiplex (multiplex number is 2, mapping two modulating symbols to one sub-carrier) is used herein. As a spread code, a Walsh code is used. A Walsh code is generated from the Hadamard matrix. The Hadamard matrix is a diametric alignment in which rows and columns are at right angles of each other with +1 and −1 as elements. The Walsh matrix can be obtained by realigning an ascending order in the number of times that a code is replaced with a column in the Hadamard matrix. The spread multiplex conversion matrix T2 supporting the duple spread and the duple multiplex is expressed as formula (1).
                              T          2                =                  (                                                    1                                            1                                                                    1                                                              -                  1                                                              )                                    (        1        )            
A spreading and multiplexing unit 102 conducts duple spread and duple multiplex according to formula (2). As a result, two modulating symbols b [2n−1] and b[2n] are outputted as two chip signals c1 [n] and c2[n].
                              (                                                                                          c                    1                                    ⁡                                      [                    n                    ]                                                                                                                                            c                    2                                    ⁡                                      [                    n                    ]                                                                                )                =                                            T              2                        ⁡                          (                                                                                          b                      ⁡                                              [                                                                              2                            ⁢                            n                                                    -                          1                                                ]                                                                                                                                                        b                      ⁡                                              [                                                  2                          ⁢                          n                                                ]                                                                                                        )                                =                      (                                                                                                      b                      ⁡                                              [                                                                              2                            ⁢                            n                                                    -                          1                                                ]                                                              +                                          b                      ⁡                                              [                                                  2                          ⁢                          n                                                ]                                                                                                                                                                                    b                      ⁡                                              [                                                                              2                            ⁢                            n                                                    -                          1                                                ]                                                              -                                          b                      ⁡                                              [                                                  2                          ⁢                          n                                                ]                                                                                                                  )                                              (        2        )            
A serial/parallel conversion unit (S/P) 103 converts two-system chip signal c1 [n] and c2 [n] as respective serial signal inputs into parallel signals. The parallel signal numbers are based on a ratio of sub-carrier numbers used for data transfer with a spread rate. In case the sub-carrier numbers are 512, for example, a spread rate is 2 and the parallel numbers are 256, which is given by a formula 512 divided by 2.
An Inverse Fast Fourier Transform (IFFT) 104 provides a paralleled chip signal hk with Inverse Fast Fourier Transform processing, and converts it as the signal in a frequency region into a signal in a time region. The chip signals c1 [n] and c2 [n] of which n are the same are herein given a distance as sufficient as possible on the frequency region. By this, higher frequency diversity effects can be obtained.
A parallel/serial conversion unit (P/S) converts a signal in a time region after output of IFFT 104 into a serial signal. A guard interval insertion unit (+GI) 106 adds a guard interval to its serial signal. A guard interval is a signal that maintains orthogonal nature between sub-carriers even at a receiving side. A signal after the addition of the guard interval is sent by radio transmission together with a pilot signal (not illustrated herein). A pilot signal is used for estimating a channel at a receiving side.
In a receiving device 200, illustrated in FIG. 13, a guard interval removing unit (−GI) 201 removes guard intervals from the signals received by radio transmission. An S/P 202 converts signals after removing guard intervals into parallel signals. A Fast Fourier Transform (FFT) 203 provides the paralleled chip signals with Inverse Fast Fourier Transform processing, and converts them from the signals in a frequency region into sub-carrier signals Hk in a time region. The sub-carrier signal Hk hereof includes changes of amplitude and a phase received in a channel.
A channel estimation and MMSE correction unit 204 measures a state of channels according to receiving characteristics of pilot signals. The unit also measures a noise power density in a frequency band. Then the channel estimation and MMSE correction unit 204 conducts an equivalent processing using Minimum Means Square Errors (MMSE) according to the channel state and the noise power density. A sub-carrier signal Hk′ output from the channel estimation and MMSE correction unit 204 is expressed in formula (3).
                              H          k          ′                =                                            H              k                        ·                          A              k              *                                                                                                            A                  k                                                            2                        +                          N              0                                                          (        3        )            
However, Hk is the Kth order of the sub-carrier signal that is input, and Hk′ is the Kth order of the sub-carrier that is output, Ak is a channel stat of the sub-carrier number K and No is a noise power density.
The P/S 205 converts the sub-carrier signal Hk into a serial signal, and outputs the signal as a combination of (c1′ [n] and c2′ [n]) the chip signal duple spread.
An inverse spreading unit 206 obtains a correlation of a chip signal and a spread code, and then restores the modulated symbol that was spread. More specifically, an operation as expressed formula (4) is arranged.
                              (                                                                                          b                    ′                                    ⁡                                      [                                                                  2                        ⁢                        n                                            -                      1                                        ]                                                                                                                                            b                    ′                                    ⁡                                      [                                          2                      ⁢                      n                                        ]                                                                                )                =                                            T              2                              -                1                                      ⁡                          (                                                                                                                  c                        ′                                            ⁡                                              [                        n                        ]                                                                                                                                                                                c                        2                        ′                                            ⁡                                              [                        n                        ]                                                                                                        )                                =                                    1              2                        ⁢                          (                                                                    1                                                        1                                                                                        1                                                                              -                      1                                                                                  )                        ⁢                          (                                                                                                                  c                        ′                                            ⁡                                              [                        n                        ]                                                                                                                                                                                c                        2                        ′                                            ⁡                                              [                        n                        ]                                                                                                        )                                                          (        4        )            
The modulated symbol spread in a chip signal c1 [n] that supports the sub-carrier signal Hk in the number K under K/2≧k in chase where the sub-carrier numbers are K is also spread into a sub-carrier signal Hk+K/2, that also supports the chip signal C2 [n]. When background noises mixed into these two sub-carrier signals Hk and Hk+k/2 are respectively nk, and nk+k/2, a formula (5) is satisfied.
                                                        H              k                        =                                                            A                  k                                ⁢                                                      c                    1                                    ⁡                                      [                    n                    ]                                                              +                              n                k                                              ,                                          ⁢                                    H                              k                +                                  K                  /                  2                                                      =                                                            A                                      k                    +                                          K                      /                      2                                                                      ⁢                                                      c                    2                                    ⁡                                      [                    n                    ]                                                              +                              n                                  k                  +                                      K                    /                    2                                                                                      ⁢                                  ⁢                                                            c                1                ′                            ⁡                              [                n                ]                                      =                                          H                k                ′                            =                                                                                                                                                                    A                          k                                                                                            2                                        ⁢                                                                  c                        1                                            ⁡                                              [                        n                        ]                                                                              +                                                            A                      k                      *                                        ⁢                                          n                      k                                                                                                                                                                                A                        k                                                                                    2                                    +                                      N                    0                                                                                ,                                          ⁢                                                    c                2                ′                            ⁡                              [                n                ]                                      =                                          H                                  k                  +                                      K                    /                    2                                                  ′                            =                                                                                                                                                                    A                                                      k                            +                                                          K                              /                              2                                                                                                                                                  2                                        ⁢                                                                  c                        2                                            ⁡                                              [                        n                        ]                                                                              +                                                            A                                              k                        +                                                  K                          /                          2                                                                    *                                        ⁢                                          n                                              k                        +                                                  K                          /                          2                                                                                                                                                                                                                              A                                                  k                          +                                                      K                            /                            2                                                                                                                                      2                                    +                                      N                    0                                                                                                          (        5        )            
When the above formulas (3) and (5) are assigned to the formula (4), demodulated symbols after inverse spread can be obtained. For example, b′ [n−1] is expressed in formula (6).
                                          b            ′                    ⁡                      [                          n              -              1                        ]                          =                                            (                                                                                                                                        A                        k                                                                                    2                                                                                                                                                        A                          k                                                                                            2                                        +                                          N                      0                                                                      +                                                                                                                          A                                                  k                          +                                                      K                            /                            2                                                                                                                                      2                                                                                                                            A                                                  k                          +                                                      K                            /                            2                                                                                                                                      +                                          N                      0                                                                                  )                        ⁢                          b              ⁡                              [                                  n                  -                  1                                ]                                              +                                    (                                                                                                                                        A                        k                                                                                    2                                                                                                                                                        A                          k                                                                                            2                                        +                                          N                      0                                                                      -                                                                                                                          A                                                  k                          +                                                      K                            /                            2                                                                                                                                      2                                                                                                                                                        A                                                      k                            +                                                          K                              /                              2                                                                                                                                                  2                                        +                                          N                      0                                                                                  )                        ⁢                          b              ⁡                              [                n                ]                                              +                      (                                                                                A                    k                    *                                    ⁢                                      n                    k                                                                                                                                                            A                        k                                                                                    2                                    +                                      N                    0                                                              +                                                                    A                                          k                      +                                              K                        /                        2                                                              *                                    ⁢                                      n                                          k                      +                                              K                        /                        2                                                                                                                                                                                                      A                                                  k                          +                                                      K                            /                            2                                                                                                                                      2                                    +                                      N                    0                                                                        )                                              (        6        )            
In the above formula (6), the first section of the right-hand member is a section related to an intended modulated symbol b [n−1], the second section is a section related to an interfering modulated symbol b [n] (the section related to interference noises), and the third section is a section related to background noises. Supposing relations of the formula (7) can be satisfied herein, the background noises are only mixed as noise components without mutual interference of the modulated symbols b [n−1] and b [n].|Ak|=|Ak+K/2|  (7)
Although relations of the above formula (7) are generally not available, electric power of an average noise (which is a combination of background noises and interference noises) is ensured to be minimized according to an operation based on the above formula (3) even in such a case. As a result, signals supporting the modulated symbols b [n−1] and b [n] are input into a demodulator (DEM) 207 as a signal point on an IQ flat surface. The modulator 207 decides that it is the receiving data that is the standard single point (refer to FIG. 14) of the modulated symbol closest to the receiving signal point. The receiving signal point, however, includes interference components mixed at the time of inverse spread, as expressed in the above formula (6). The receiving accuracy, thus, becomes deteriorated in the modulator 207. (See: N. Miyazaki and T. Suzuki, “A Study on Forward Link Capacity in MC-CDMA Cellular System with MMSEC Receiver,” IEICE Trans. Commun., Vol. E88-B, No. 2, pp. 585-593, February 2005.)
As described above, a conventional MC-CDM system cannot avoid deterioration of receiving characteristics affected by inter-code interference caused by frequency selectivity of a radio channel, even in case where an MMSE-based equalization technology, in which the characteristics are believed to be most excellent, is applied.